CHAPTER 9 OTHER FLATFISH by Paul D. Spencer, Gary E. Walters, and Thomas K. Wilderbuer EXECUTIVE SUMMARY The other flatfish species complex had previously included Alaska plaice, which are assessed with an age-structured population model. Because the 2002 harvest specifications separated Alaska plaice from the other flatfish complex, the assessment of Alaska plaice has been presented as a separate chapter in this SAFE document. The assessment of the remaining other flatfish, excluding Alaska plaice, is presented in this chapter. The following changes have been made to this assessment relative to the November 2001 SAFE: Changes in the input data 1) The 2001 catch (total and discarded) was updated, and catch through 28 September, 2002 were included in the assessment. 2) 2002 trawl survey biomass estimates and standard errors of other flatfish species were included in the assessment. Changes in assessment methodology 1) The tier 4 approach in previous assessments of applying the flathead sole F40 and F35 fishing rates is examined with respect to the available data for the other flatfish complex, and it is recommended that this complex be assessed under tier 5. A natural mortality rate of 0.20 was used for the tier 5 calculations, based upon values used in age-structured analyses of BSAI flatfish. A summary of the harvest recommendations for 2003 (under tier 5) is compared to the tier 4 recommendations used in the 2002 assessment. 2001 Assessment 2002 Assessment recommendations recommendations (Tier 4) (Tier 5) Exploitable biomass 78,293 t 106,739 t ABC 18,065 t 14,691 t Overfishing 21,832 t 19,588 t FABC F0.40 = 0.30 0.15 Foverfishing F0.35 = 0.38 0.20 INTRODUCTION Prior to 2001, the “other flatfish” species complex included Alaska plaice (Pleuronectes quadrituberculatus). Flathead sole (Hippoglossoides elassodon) were part of the other flatfish complex until they were removed in 1995, and in recent years Alaska plaice was the dominant species of the complex and comprised 87% of both the 2000 catch and the estimated 2001 trawl survey biomass. Because more biological information exists for Alaska plaice than for the remaining species of other flatfish, an age-structured population model was used to assess this stock. In contrast, survey biomass estimates are the principal data source used to assess the remaining other flatfish. In 2002, Alaska plaice were and removed from the other species complex and managed separately. Given the differences in biological information, assessment techniques, and management, it is appropriate to separate the assessment of Alaska plaice from the remaining other flatfish. This chapter considers the assessment of other flatfish excluding Alaska plaice. Catch History The miscellaneous species found in the other flatfish species category are listed in Table 1, and their catches from 1995-2002 are shown in Table 2. These catch estimates were produced by applying the proportional catch , by species, from fishery observer data to estimates of total catch for the other flatfish complex. In recent years, starry flounder (Platichthys stellatus) and rex sole (Glyptocephalus zachirus) account for most of the harvest of other flatfish, and contributed 89% of the harvest of other flatfish in 2002. Other flatfish are grouped with Alaska plaice, rock sole, and flathead sole and other flatfish fisheries in a single prohibited species class (PSC) classification, with seasonal and total annual allowances of prohibited bycatch applied to the classification. In recent years, this group of fisheries has been closed prior to attainment of the TAC due to the bycatch of halibut (Table 3), and portion of the eastern Bering Sea has been closed to these fisheries in 2002 for exceeding the red king crab bycatch allowance. DATA Absolute Abundance and Exploitation Rates The biomass of the other flatfish complex on the eastern Bering Sea shelf has been relatively stable from 1983-1995, averaging 52,000 t, and has slightly increased from 1996 to 2002, averaging 75,000 t (Table 4). The 2002 biomass estimate of other flatfish on the EBS shelf is 97,938 t. Increases in biomass have also been seen in the Aleutian Islands trawl survey; the 2002 estimate is 8800 t. Individual species biomass estimates from 1997-2002 are shown in Table 5. Exploitation rates for starry flounder and rex sole have been low, not exceeding 0.10 from 1997 to 2001 (Table 9). The exploitation rates for butter sole have been slightly higher, exceeding 0.15 in 1997, 2000, and 2001, but the biomass estimates for butter sole have large sampling variances, with coefficients of variation ranging from 0.5 to 0.64 in recent EBS trawl surveys. PROJECTIONS AND HARVEST ALTERNATIVES Reference Fishing Mortality Rates and Yields The other flatfish complex are currently managed under Tier 4 of Amendment 56 to the BSAI groundfish management plan, and thus rely upon knowledge of maturity ogives, growth rates, and fishery selectivity in order to produce a F40% estimate. In previous assessments, the F40% estimate from flathead sole was used as a proxy for this complex. This practice originated before Alaska plaice were assessed with an age-structured model, as flathead sole were viewed as a reasonable proxy for Alaska plaice. Starry flounder are currently the dominant component of the other flatfish complex, it is useful to reexamine the appropriateness using flathead sole as a proxy species. Although relatively little information exists on starry flounder life-history characteristics in Alaska, the information available suggest potentially considerable differences from flathead sole in growth and maturity. Hart (1973), citing California studies, indicates that females mature at age 3 at around 350 mm; in contrast, female flathead sole in Alaska grow to about 160 mm at age 3 and have a 50% maturity at approximately age 8 (although the size at maturity is similar). Despite the lack of information about starry flounder in Alaska waters, use of an F40% proxy could be justified if there was little variation in growth and maturity patterns in flatfish for which we have more information. However, F40% levels in relatively small flatfish (yellowfin sole, rock sole, Alaska plaice, and flathead sole) differ by a factor of three, ranging from 0.11 for yellowfin sole to 0.30 for flathead sole. The yellowfin sole F40% estimate is lower than other flatfish species because of slower growth and delayed maturity schedules, thus making them more vulnerable to overfishing in mixed-species trawl fisheries (Spencer et al. 2002). In conclusion, it appears that the use of a flathead sole F40% proxy for other flatfish is not supported by the available data, and it is recommended that this complex be assessed with tier 5 methods. The information requirements for tier 5 assessments are estimates of biomass and natural mortality. The natural mortality rates used in age-structured BSAI flatfish assessments can be used as guidance and are presented below: Species Natural mortality rate used for stock assessment Yellowfin sole 0.12 Rock sole 0.18 Flathead sole 0.20 Alaska plaice 0.25 Given this range of values, an assumption of 0.20 appears reasonable. The estimates of Fabc and Fofl under tier 5 are 0.75M and M, respectively, and the ABC and OFL levels are the product of the fishing mortality rate and the biomass estimate. Given the Fabc and Fofl levels of 0.15 and 0.20, and the biomass estimate of 106,739 t, the resulting ABC and OFL levels are 14,691 and 19,588 t. These values are more conservative than those obtained from the tier 4 approach of applying (using the catch equation) the F40% and F35% levels estimated from this years (2002) flathead sole assessment to the 2002 BSAI survey biomass of miscellaneous flatfish. The 2002 estimates of F40% and F35% for flathead sole are 0.286 and 0.355, respectively, and the tier 4 ABC and OFL levels are 23,649 t and 28,440 t. A comparison of the tier 4 and tier 5 estimates is shown below: F level (value) Projected yield for year 2003 Tier 4 FABC (0.286) 23,649 t Tier 4 FOFL (0.355) 28,440 t Tier 5 FABC (0.15) 14,691 t Tier 5 FOFL (0.20) 19,588 t In conclusion, it is recommended that the other flatfish complex be assessed under tier 5. REFERENCES Hart, J.L. 1973. Pacific fishes of Canada. Fisheries Research Board of Canada, Bulletin 180, Ottawa. 740 pp. Spencer, P.D., T.K. Wilderbuer, and C.I. Zhang. 2002. A mixed-species yield per recruit model for eastern Bering Sea flatfish fisheries. Can J. Fish. Aquat. Sci. 59:291-302. Table 1. Flatfish species of the Bering Sea/Aleutian Islands “other flatfish” management complex. Common Name Scientific Name Arctic flounder Liopsetta glacialis butter sole Isopsetta isolepis curlfin sole Pleuronectes decurrens deepsea sole Embassichths bathybus Dover sole Microstomus pacificus English sole Parophrys vetulus longhead dab Limanda proboscidea Pacific sanddab Citharichthys sordidus petrale sole Eopsetta jordani rex sole Glyptocephalus zachirus roughscale sole Clidodoerma asperrimum sand sole Psettichthys melanostictus slender sole Lyopsetta exilis starry flounder Platichthys stellatus Sakhalin sole Pleuronectes sakhalinensis Table 2. Harvest (t) of other flatfish from 1995-2002. Starry Rex Butter Remaining Year Founder Sole Sole Species Total 1995 337 512 163 15 1027 1996 1194 984 219 98 2495 1997 1193 588 492 179 2451 1998 330 775 214 41 1359 1999 756 655 213 16 1640 2000 1012 748 349 20 2129 2001 644 682 198 18 1542 2002* 1120 1053 205 32 2441 *NMFS Regional Office Report through Sept 28, 2002 Table 3. Restrictions on the “other flatfish” fishery from 1994 to 2001 in the Bering Sea – Aleutian Islands management area. Note that in 1994, the other flatfish category included flathead sole.